一时 has several nuances: (1) 'at the moment' - 一时想不起来 (can't think of it right now); (2) 'on impulse' - 一时冲动 (impulsive moment); (3) 'temporary' - 一时的困难 (temporary difficulty). Context determines the meaning.
usage
一时半会儿 = in a short while (usually negative: can't do something quickly). 一时兴起 = on a whim.
